A 3 year old may feel bad or ashamed when asking for a diaper change as they because they are embarrassed about not being able to use the toilet and embarrassed about having poop or pee in their diaper.
By 3 years old you can get embarrassed and ashamed and so some 3 year olds don't like wearing diapers or being changed and are ready to be potty trained.
If a 3 year old is embarrassed or feels bad or ashamed about wearing diapers or asking for a diaper change then they should start potty training so they won't have to wear the diaper anymore.
However the 3 year old should not have to ask for diaper change as the parent should check the diaper and change it when needed.
Also give the 3 year old some privacy when they get their diaper changed and take them into the bathroom, bedroom etc or somewhere private.
Toddlers use diapers even when they can hold it because toddlers have been so used to wearing and using diapers since birth.
Since the the toddler was born they were put in a diaper and were supposed to use the diaper to pee and poop in and so suddenly the toddler is expected to not use the diaper and use the toilet instead.
The toddler uses the diaper as it's a habit they were so used too so you have to break that habit by reminding the toddler to use the toilet instead of the diaper.
Still if the toddler is not fully ready and still uses the diaper instead of the toilet then you should keep them in diapers for awhile longer and try again with toilet training later when the toddler is more ready.
The point of making your kids wear a diaper is to keep the kids clothes clean and also to keep your furniture, carpets etc clean.
If a kid who is not potty trained does not wear a diaper then they will pee and poop in their pants and get it all over them, the bed, your couch, the floor, in your car etc.
Without a diaper on the kid who's not potty trained there would be a huge mess and it would be unhygienic and unhealthy for the kid to be peeing and pooping on themselves and in their clothes.
When babies are born they have no bladder or bowel control and even as the child gets older and reaches 2 to 3 years old the child's bladder is still weak so they can't hold it very long.
So a child who is wetting their pants or pooping their pants or those who are not potty trained will need to wear a diaper.
Even if the kid does not want to wear the diaper they must be made to wear the diaper if they are not potty trained.
Or if a kid is incontinent or bed wetting then they should wear a diaper to keep them healthy and clean.
Some kids potty train by age 18 months and some are not ready until 2 to 3 years old.
Then again some kids may not be potty trained until 4 and even 5 years old.
Every kid is different and will eventually potty train and want to stop wearing diapers but until then the child must and should wear a diaper.